Recently, cassette tape recorders have been reduced both in size and in weight, and accordingly reduced size magnetic tape cassettes are required. In the field of audio technique, there has been a strong demand for the provision of magnetic tape cassettes which can record and reproduce signals with high quality and high density over a long period of time. In order to meet these requirements, a magnetic tape cassette has been proposed which operates in accordance to a digital system such, as a pulse code modulation (PCM) system, in which the pulse conversion of input signals is carried out for recording and reproducing operations, has been proposed. One example of a magnetic tape cassette operated according to such PCM system is a large video tape cassette which has a recording frequency band which is wider than an audio compact cassette. It's recording frequency band should be about five times as wide as that of a conventional audio magnetic tape cassette.
The video tape cassette is generally for a rotary head system, and has a guard panel which is adapted to close an opening formed in the front wall of the cassette and which guard panel can be swung up and down. That is, a magnetic tape cassette operated according to the digital system is purposely for signal recording and reproducing operation of short recording wavelength, high density, and wide frequency band. Therefore, the structure of the cassette should be designed so that the cassette adequately prevents dust from entering the cassette interior and wherein the tape is also adequately protected from damage, in comparison with that of conventional audio compact cassette. Since it is estimated that the frequency of use of the magnetic tape cassette outdoors, increases day by day, similar to the case of a conventional audio compact cassette, it is essential that the cassette constitute an excellent dust proof structure.
In view of the foregoing requirements, a magnetic tape cassette has been proposed which is so designed that when it is not in use, the opening formed in the front wall of the cassette and the reel shaft inserting holes are closed or sealed to prevent the entrance of dust into the cassette interior. The cassette is provided with a slide guard which is slidable on the bottom of the lower half case of the cassette. The slide guard is urged by a spring or like biasing member, such that when the cassette is in use, the lower part of the opening at the end of the cassette case is opened and the reel shaft inserting holes are exposed. To the contrary, when the cassette is not in use, the lower part of the opening within the front of the cassette case and the reel shaft inserting holes are closed. The lower half case has a cantilever type locking member having a free end part which normally protrudes below the bottom of the lower half case. The end part of the locking member is adapted to engage selectively, two locking holes formed in the slide guard. The end part of the locking member engage may one of the two locking holes so that the slide guard is fixedly positioned in place, after being slid into position beneath the lower half case. The magnetic tape recording and reproducing device is provided with a disengaging member which is adapted to push the end part of the locking member against the bottom of the bucket which is the cassette loading section, thereby disengaging the end part from the slide guard locking hole. That is, when the cassette is loaded, the means for maintaining the opening within the end of the cassette case closed is released by the disengaging member; and when the case is unloaded, the means for maintaining the opening open is released by the disengaging member.
In assembling the cassette, the slide guard is attached to the lower half case in such a manner that it is slidable along the lower half case. However, since the end part of the locking member protrudes below the bottom of the lower half case as described above, the end part of the locking member abuts against the leading edge of the slide guard as the latter is moved backwardly of the cassette lower half case, thus obstructing the attaching of the slide guard to the lower half case or the sliding movement along the case. Accordingly, the attaching of the slide guard to the lower half case is considerably intricate and the action rendered complicated, because it is necessary to disengage the end part of the locking member from the edge of the slide guard in order to permit the slide guard to move from the front to the rear of the lower half case. In order to eliminate this difficulty, a system may be employed in which, when the slide guard is attached to the lower half case, the locking member is mechanically bent so that the end part thereof is caused to move inwardly of the lower half case. However, such an arrangement causes other difficulties such as the locking member being broken when bent excessively, and the elasticity decreasing by creep so that the locking effect decreases over time.